1. Field of the Invention
This invention relates to a pressure relief assembly for protection against potentially dangerous fluid pressure buildup within an enclosure, wherein the assembly comprises a first frangible portion rupturable at a first, certain pressure to present a venting aperture having a first cross-sectional area, as well as a second frangible portion rupturable at a pressure higher than the first pressure to present a pressure relieving aperture of a cross-sectional area larger than the aperture presented on rupture of the first frangible portion. Staged release of the two frangible portions at different pressures amply vents the enclosure with a minimum of fluid turbulence and, where the enclosure partially contains liquids, substantially reduces the likelihood of two-phase outflow of the pressurized contents.
2. Description of the Prior Art
Devices for relieving pressure within a fluid vessel, piping or other types of structure often include sheets, panels or discs that are frangible and rupture or burst at a pressure which is low enough to prevent explosion of the vessel or other enclosure defining structure. Such frangible members can take the form of rupture discs or burst panels; in any case, frangible venting assemblies have been known to provide a predictable relief area upon initial rupture at a specified level of pressure.
Examples of rupture discs known in the art are found in U.S. Pat. Nos. 4,512,491, dated Apr. 23, 1985, 4,505,180 dated Mar. 19, 1985 and 3,109,553 dated Nov. 5, 1963, all of which are assigned to the assignee of the present invention. Such rupture disc assemblies typically comprise either a single frangible disc or a pair of frangible discs mounted in face-to-face, spaced relation, and often the discs are provided with lines of weakness to provide a more predictable and instantaneous response to bursting at a given pressure level internally of a vessel to be protected. The lines of weakness enable the disc to fully open when the design pressure is reached, so that a maximum of vent cross-sectional area enables the buildup of fluid pressure within the vessel to be quickly relieved.
Other types of frangible venting assemblies include burst panels of the type shown in U.S. Pat. No. 4,067,154, dated Jan. 10, 1978 and also assigned to the assignee of the instant invention. Burst panels are commonly used to protect pressure buildup in equipment such as bag houses, cyclone-type separators and other dust collecting equipment which typically operate at pressures lower than vessels protected by rupture discs. Burst panels are often provided with score patterns, perforations or other types of weakness lines so that the panel bursts with a relatively high degree of predictability at a certain design pressure.
While the frangile pressure relief assemblies described in the aforementioned U.S patents provide a valuable function by protecting both equipment as well as personnel, it is believed that in certain circumstances it would be desirable to avoid fully opening the vent area during the initial stages of pressure buildup. For example, in the case of burst panels for explosion vents, it has been observed that the large opening presented by the burst panels after rupture of the latter causes gas to escape through the vent at a relatively large rate of flow, and unfortunately the large outflow of gas in the vessel occasionally enhances turbulence which in turn undesirably accelerates the combustion process.
Similarly, rupture disc assemblies, such as those used to protect liquid-containing vessels which can be subject to pressure, are constructed to fully open at a certain design pressure that is substantially below the burst pressure of the vessel. However, if the liquid within the vessel is exposed to pressures greater than atmospheric pressure, the liquid may undergo rapid depressurization as soon as the rupture disc assembly opens. During this process, bubbles sometimes form within the bulk of the liquid to re-establish equilibrium of pressure between the liquid and the gas, and consequently during rapid depressurization of the vessel, the liquid will froth up suddenly, causing the liquid and gas to escape together through the vent opening provided by the rupture disc assembly. Such two-phase flow is dangerous to operating personnel and also in some instances represents a potentially costly loss of materials.
It has also been proposed in the past to provide two separate rupture disc assemblies at spaced locations on a vessel to be protected, wherein a disc of one of the assemblies is constructed to burst at a first pressure and a disc of the other assembly is intended to rupture at a second, higher pressure in an effort to provide a failsafe, "backup" system for relieving pressure in the vessel in instances where the first assembly for some reason fails to rupture. However, such dual systems must incorporate two separate piping means for discharging the outflowing liquid and gas, thereby increasing the overall cost of the system. Moreover, provision of two separate rupture disc assemblies increases the likelihood of leakage and more or less doubles the expenditures for installation and maintenance.
As can be appreciated, it would be a desirable advance in the art to provide a pressure relief assembly for vessels, fluid conduits as well as other types of equipment subject to fluid pressure buildup which would avoid the problems presented hereinabove. Explosion vents, for instance, would desirably be constructed to avoid contributing turbulence to the combustion process within the vessel. Also, rupture disc assemblies would advantageously be constructed to substantially preclude two-phase flow of contents through the vent opening of reactor vessels, tanks and the like.